vdpa/mlx5: make MTU/STATUS presence conditional on feature bits

The spec says:
    mtu only exists if VIRTIO_NET_F_MTU is set
    status only exists if VIRTIO_NET_F_STATUS is set

We should only present MTU and STATUS conditionally depending on
the feature bits.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Message-Id: <1675725124-7375-6-git-send-email-si-wei.liu@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>